Alternative refrigerants such as HFC-125 (C2HF5) and HFC-32 (CH2F2) have been widely used as important replacements for CFC, HCFC, etc. that cause ozone layer depletion. However, these alternative refrigerants are potent global warming substances, thus creating concern that diffusion of the refrigerants would enhance global warming. As a preventive measure, these refrigerants are recovered after use. However, complete recovery of the refrigerants is impossible. In addition, the diffusion of said refrigerants due to leakage, etc. cannot be ignored. The use of CO2 or hydrocarbon-based substances as alternative refrigerants has also been investigated. However, because CO2 refrigerants have low efficiency, devices using such refrigerants inevitably become large. Thus, CO2 refrigerants have many problems in terms of the overall reduction of greenhouse gas emissions, including energy to be consumed. Furthermore, hydrocarbon substances pose safety problems due to their high flammability.
2,3,3,3-tetrafluoropropene (HFO-1234yf, CF3CF═CH2), which is a olefinic HFC having a low global warming potential, has recently been attracting attention as a material to solve the above problems.
A known method for producing 2,3,3,3-tetrafluoropropene (HFO-1234yf) comprises subjecting 1,1,1,2,3-pentafluoropropane (HFC-245eb) or 1,1,1,2,2-pentafluoropropane (HFC-245cb) to a dehydrofluorination reaction. However, when such a dehydrofluorination method is used, HFO-1234yf is obtained as a mixture of HFO-1234yf with HF, and it is thus necessary to remove HF by using some method.
The simplest method for removing HF from a mixture of HFO-1234yf and HF is using water to absorb HF. However, HFO-1234yf treated by this method is always intermixed with steam mist, or water in an amount corresponding to the vapor pressure. There are also various other sources of water, such as water contained in the starting materials, water derived from the catalyst, and water remaining in the equipment. Thus, the obtained HFO-1234yf is intermixed with these waters.
The water contained in the final product HFO-1234yf affects the performance of HFO-1234yf as a refrigerant, as well as its stability and corrosivity to the device. Therefore, the removal of water is an important factor in quality control, and a method for removing water is a particularly important technique.
A common method for removing water is the use of an adsorbent, such as a molecular sieve. For example, Patent Literature (PTL) 1 listed below discloses a method comprising drying liquid HFO-1234yf over zeolite. However, this method requires the high-speed treatment of a gas having a comparatively low water content, thus necessitating the use of a large dehydrating tower. Furthermore, periodic reactivation or replacement of the adsorbent is also necessary, and industrial waste is generated in large amounts during the replacement of the adsorbent.